Cathode-ray tube



Jan. 28, 1941. F. NlCOLL ETAL CATHODE RAY TUBE Filed Sept. 13, 1938 INVENTORS v FREDERICK HERMES NICOLL AND BERNARD JOSEPH -MA Y0 Patented Jan. 28, 1 941 UNITED STATES PATENT OFFICE CATHODE-RAY Application September 13, 1938, Serial No. 229,724 In Great Britain September 13, 1937 2 Claims.

This invention relates to high vacuum cathode-ray tubes and is concerned with the electrode system of such tubes known as the electron gun.

The electron gun of a cathode-ray tube commonlyconsists of a cathode, an initial focussing and modulating electrode and a combination of accelerating electrodes which focus the electron beam onto for example, the fluorescent screen of the tube.

It is found that in operation of such tubes the diameter of the spot whichis finally focussed on to the screen of the tube varies as the modulation potentials applied to the tube vary. This, it will be appreciated, is undesirable and it is the object of the present invention to provide an improved cathode-ray tube and a method of operating such a tube with a view to reducing the variation in diameter of the spot which occurs as the modulation potentials vary.

According to one feature of the invention a method is provided of operating a high vacuum or hard cathode-ray tube which has within its envelope in the order named, a cathode, an apertured cathode screen, a first accelerator, a decelerator or constricting electrode, a second accelerator or first anode, and an electron receiving target or screen, and means, not necessarily Within the envelope, for finally focussing a beam of electrons from said cathode onto the screen of the tube, the method consisting in accelerating and causing the electrons to cross over between the cathode and the first accelerator, decelerating the cathode ray beam to form a second cross-over point near to the end of said first anode facing the cathode, modulating the beam in the region of one of said cross-overs and accelerating the electron beam and focussing an image of the said second cross-over point onto said screen the method being such that the diameter of the focussed spot on the screen is rendered more nearly constant.

According to a further feature of the invention a circuit arrangement is provided embodying a cathode-ray tube of the high vacuum or hard type having arranged within its envelope in the order named, a cathode, an apertured cathode screen, an apertured' accelerator electrode, a decelerating electrode comprising an apertured diaphragm, a second accelerator or first anode, and an electron receiving target, and means, not necessarily within the envelope for finally focussing the beam into a small spot on the target of the tube wherein signals to be reproduced on said target are applied either to modulate the beam between said cathode and first accelerator or to modulate the beam at said decelerating electrode, and wherein means are provided for applying to the first accelerator a high potential positive with respect to the cathode and a potential to the decelerator which is the same as, or slightly positive or negative with respect to, the cathode potential, and a high potential positive with respect to the cathode to the screen accelerator, the arrangement of the electrodes and the potentials applied thereto being such that, in operation, the electrons emanating from the cathode are caused to cross over between the cathode and first accelerator and are caused to cross over' again near the end of the second accelerator or first anode facing the cathode, the arrangement being further such that the variation of the size of the spot on said target over the higher range of current densities is substantially less than the variation represented in curve A of Figure 2 of the drawing hereinafter referred to. 7

According to a further feature of the invention a circuit arrangement is provided embodying a cathode-ray tube of the hard type having arranged within its envelope in the order named, a cathode, an apertured cathode screen, an apertured accelerator electrode, a decelerating electrode comprising an apertured diaphragm, a second accelerator or first anode, and an electron receiving target, and means, not necessarily within the envelope, for-finally focussing the beam into a small spot on the screen of the tube, the first accelerator being disposed from the cathode a distance of between 3 and 5 times the diameter of the aperture in the cathode screen, and wherein signals to be reproduced on the said target are applied either to modulate the beam between said cathode and first accelerator or to modulate the beam at said decelerating electrode, and wherein means are provided for applying to the first accelerator a high potential positive with respect to the cathode and a potential to the decelerator which is the same as or slightly positive or negative with respect to the cathode potential and a high potential positive with respect to the cathode to the second accelerator, the arrangement of the electrodes and the potentials applied thereto being such that in operation the variation of the size of the spot on said target over the higher range of current densities is substantially constant, that is, less than the variation represented in curve A of said Figure 2 of the drawing.

According to another feature of the invention a cathode-ray tube of the hard type is provided having arranged within its envelope in the order named, a cathode, an apertured cathode screen, an apertured accelerator electrode, a decelerating electrode comprising an apertured diaphragm, a second accelerator or first anode, and a target, and means, not necessarily within the envelope, for finally focussing the beam into a small spot on the target of the tube, the shape and disposition of the various electrodes being such that when in operation a relatively high potential positive with respect to the cathode is applied to said first accelerator and av potential is applied to the decelerator which is the same as, or slightly positive or negative with respect to, the cathode potential, and a high positive potential with respect to the cathode is applied to the second accelerator, and when the cathode-ray beam is modulated between said cathode and first accelerator the electrons emanating from the cathode are caused to cross over between the cathode and first accelerator and are caused to cross over again near to the end of the second accelerator or first anode facing the cathode, the. arrangement being such that the diameter of the focussed spot on the target of the tube is rendered substantially constant as the current in the cathode-ray beam is varied.

According to another feature of the invention a cathode-ray tube of the hard type is provided having arranged within its envelope in the order named, a cathode, an apertured cathode screen, an apertured accelerator electrode, a decelerating electrode comprising an apertured diaphragm and a second accelerator or first anode, and means, not necessarily within the envelope, for finally focussing the beam into a small spot on the screen of the tube, the first accelerator being disposed from the cathode a distance of between 3 and 5 times the diameter of the aperture in the cathode screen and the arrangement and disposition of the electrodes is such that when in operation a relatively high potential positive with respect to the cathode is applied to said first accelerator and a potential is applied to the decelerator which is the same as or slightly positive or negative with respect to the cathode potential and a high positive potential with respect to the cathode is applied to the second accelerator and when the cathode-ray beam is modulated between said cathode and said first accelerator the diameter of the focussed spot on the screen of the tube is rendered more nearly constant as the current in the cathode-ray beam is varied.

In order that the nature of the invention may be more clearly understood, an electron gun system designed in accordance therewith will now be described by way of example with reference to the accompanying drawing in which:

Figure 1 shows the elements of such a system,

Figure 2 shows curves which demonstrate the relation between spot diameter and second anode current in the case of known tubes, and in the case of a tube according to the invention,

Figure 3 illustrates an electron gun according to a modified form of the invention and,

Figure 4 illustrates a cathode-ray tube embodying the arrangement shown in Figure 3 with the electrodes connected in circuit with appropriate sources of potential.

Referring to Figure 1 of the drawing, a cathode I is situated behind an apertured diaphragm 2 in a cylinder 3 which may be termed the cathode screen. Next to this screen is arranged a flat apertured accelerating electrode 4 which is followed by a relatively short tubular electrode 5 having an apertured diaphragm 6 arranged within it at a short distance from the accelerating electrode 4. The electrode 5 can be regarded as a constricting electrode since, owing to the potential applied thereto it constricts the field in its vicinity and causes the electrons to cross-over. The electron gun assembly is completed by the provision of a first cylindrical anode 'I of the same diameter as the cathode screen 2 and constricting electrode 5, and provided with apertured diaphragms 8 and 9, a second anode In of greater diameter than the anode I, being arranged to overlap that anode.

Assuming that the electrodes 4, 5 and I are cylindrical and of about one half inch in diameter, the operating voltages for the various electrodes of the gun assembly may be as follows. Thus, while the cathode I is maintained at zero potential the amplified picture signals or modulating potentials are preferably applied to the cathode screen electrode 3, and may vary from about -40 volts to zero. The accelerator electrode 4 is maintained at approximately 1,500 volts, but the constricting electrode 5 is maintained at zero potential or at a slightly positive potential. If the constricting electrode 5 were at a negative potential, a halo would surround the focussed spot, and this halo is removed by arranging that this electrode is at zero or at a slightly positive potential. If it is found that a halo can be tolerated, the electrode 5 may in this case be maintained at a slightly negative potential. The first anode I is maintained at a positive voltage from about 50 volts upwards and the second anode Ill is maintained at a higher positive potential than that applied to the anode I, in order that a focussing action will be produced, which will result in the electron beam being focussed to a small point on the electron receiving target I2 or screen of a cathode-ray tube containing the electron gun, as shown in Figure 4. The focussing action may alternatively be obtained by means of an electromagnetic coil, in which case the anodes 'I and I may be connected together or anode I0 may be omitted.

The apertures in the electrodes 2, l, 6 and 8 may be 0.075" in diameter. The distance between the accelerator electrode 4 and the cathode I is critical and is between three and five times the diameter of the aperture in the screen 2 the optimum distance being about four times the diameter of said aperture. The actual spacing of the electrodes may be as follows:

The screen 2' is placed in front of the cathode which is 6 mm. in diameter a distance of 0.3 mm.

whilst the tubular portion 3 is '7 mm. The accelerator 4 is separated on each side from the electrodes 3 and by 1 mm. The electrode 5 is mm. long and the diaphragm 6 spaced by the end of 5 adjacent accelerator 4 a distance of 3 mm. The anode I is 8 cms. long and spaced from the electrode 5 by 1 mm. with the diaphragm 9 spaced from the end of the anode I adjacent the second anode II] a distance of 2.5 cms. of the various electrodes may be 0.008" thick.

The electron gun shown operates in the following manner. Electrons from the cathode I are accelerated by the electrode 4, and are converged so that they cross over near the cathode and in front of the apertured diaphragm 2. The cross section of the beam at the cross-over point is small in the case of low current densities, but at high current densities electron repulsion caused The metal by space charge results in the cross section at the 76 cross-over point being considerably increased in size. The electrostatic lens between the electrode 3 apertured accelerating electrode 4 and the constricting electrode 5 with its apertured diaphragm 6, forms a one to one image of the cross section of the beam at the first crossover point, somewhere in the neighbourhood of the aperture in the diaphragm 6. The second cross-over point, however, differs from the first, in that its cross section remains small even at high current densities, because of the constricting action of the saddle shaped converging field in the electrode 5 and aperture of the diaphragm 6. A somewhat magnified image of the cross section of the beam at the point of the second cross-over is reproduced upon the electron receiving target or screen.

' When the electrode operating potentials are as indicated the operating currents, assuming small aperture diameters in the diaphragm 2, 4, 6 and 8, equal to .075", and an aperture in the diaphragm 9 of approximately 0.3, are as follows: The total emission will be of the order of 1 to 1.5 mi1liamps., the accelerator electrode (4) current 5%, the constricting electrode (5) current Zero and the first anode current 30 to 40%.

While a tubular electrode 5 and a diaphragm 6 have been shown for producing the converging saddle shaped field a differently shaped electrode may be placed between the accelerating electrode and the first anode provided that the desired converging field is produced when a potential lower than that of either the accelerating electrode 4 or the first anode 1 is applied to it.

As previously stated the modulating potentials are preferably applied to the electrode 3, these potentials being effective upon the electron beam at an appreciable distance from the constricting field, due to the electrode 5. It may, however, be advantageous in cases where the modulating potentials are. applied to the cathode screen to apply a varying potential to the constricting electrode, the varying potential having some definite phase relation to the modulating potentials. Such a varying potential would not have the elfect of varying the second anode current, and the object of applying such a varying potential would be to constrict the beam to an increasing extent with increase in the repulsion effect, due to space charge, that is to say with increasing current.

If desired, the modulation potentials may be applied to the constricting electrode 5. This is not so satisfactory as applying the modulation to the cathode screen 3 owing to the possible production of a halo. Where halo can be tolerated it is therefore immaterial whether the modulation potentials are applied to the electrode 3 or 5.

The diameter of the focussed spot on the fluorescent or other screen of a cathode-ray tube employing an electron gun embodying the invention increases more slowly with increase in current than in the case of existing types of tube, such as triodes and hexodes, the latter type being described in the Specification of the British Patent No. 431,327.

Figure 2 of the drawing shows graphically the relation between increase in the spot diameter (SD) in millimetres with increasing second anode current AC2 in micro-amperes. Thus the curve A shows this relationship in the case of a hexode tube, and the curve B shows this relationship in the case of a tube having an electron gun embodying the present invention. The curve for a triode is of the same general shape as curve A but the spotdiameter increases more at the higher range of currents.

The beneficial effect of the introduction of the constricting electrode in accordance with the present invention can be demonstrated in the following manner. If the accelerating electrode 4, the constricting electrode 5 and the first anode are all connected together andmaintained at the potential of the anode I, the electron gun functions as a triode andwhen an appropriate potential is applied to the second anode l0, an image of the cross section of the electron beam at the first cross over point is focussed on the target or fluorescent screen. A curve similar to A will then apply to suchan arrangement. The first cross over point which is focussed on the screen in this case is further from the final lens formed between the first and second anodes than is the second cross over point in an electron gun embodying the invention and may, therefore, be

expected to produce a smaller spot on the screen owing to the reduced magnification. In actual fact, however, the focussed spot has a larger diameter than when the constricting electrode is used, in accordance with the present invention.

A modified electron gun system designed for carrying'out the invention is shown in Figure 3. The dimensions of the electrodes are similar to those shown in Figure 1, that is to say, the diame- I ters of the electrodes 3 and 5 and the first part of the anode I are one half an inch in diameter or of that order. The anode! is provided at its end with a portion 1a of larger diameter, actually about one inch and a quarter, and the diameter of the second anode II] is about one inch and a half.

The cathode I is supported so that its emitting surface is .3 mm. behind the diaphragm 2 and the portion of the cylinder 3 between this diaphragm will be seen in the present case that the tubular portion of the constricting electrode 5 beyond the diaphragm B in the direction of the fluorescent screen in the construction shown in Figure 1, is omitted in the modified construction. The distance between the first anode 7 and the diaphragm 6 is thus reduced and in the modified construction a distance of 1 mm. separates the diaphragm 6 and the diaphragm 8. The tubular portion of the electrode 5 extending towards the cathode is 3 mm. long. The tubular portion of the anode 1 between the diaphragm 8 and the diaphragm 9 is 5.5 cms. long and the portion between the diaphragm 9 and the end of portion 1a is 2.5 oms. The diameters of the apertures in the diaphragms 2, 4, 6, 8 and 9 and the ,metal thickness are as stated with reference to Figure 1.

In operation, the cathode I is maintained at zero potential and the modulating potentials are applied to the cylinder 3 or electrode 5 as stated above. The voltage applied to the accelerator electrode 4 is 2000 and the constricting electrode 5 is, as in the previous case, at zero potential or thereabouts. The voltage of the anode I may be between 300 and 700 volts and that of the second anode between 3000 and 7000 volts.

It will be seen that the formation of the electron lenses and their effect upon the electron may occur within arm the neighbourhood of the saddle-shaped field due to the electrode 5 and may occur at a short distance within the anode 1. As a general rule in the construction shown in Figures 1 and 3 the distance apart of the two cross-over points should be between 3 and 20 times the diameter of the aperture in the screen 2.

If desired, the final focussing of the beamon to the screen may as stated above be effected or assisted by magnetic focussing means disposed externally of the tube.

It is immaterial whether the cathode screen during the application of modulation potentials takes currenti. e. the voltage swing causes the screen to become slightly positive with respect tothe cathode, in. cases where the diameter of the aperture in the cathode screen is smaller than the diameter of the emitting area of the cathode.

By the term cathode screen is meantan apertured electrode arranged close to the cathode and which is designed so'that when in operation it is maintained at a potential at or near thecathode potential the electrons emanatingfrom, the cathode are caused to converge.

Figure 4 illustrates the electrode structure shown-in Figure 3 mounted within an evacuated envelope I I the beam being focussed on to a screen or target l2 which is rendered luminous under impact of electrons, the target l2 being preferably a fluorescent screen. The various electrodes are energised by the potential sources shown, the signals to be reproduced on the target being applied to, the electrode 3 from the tele- 5 visionreceiver shown. .3

We claim:

1. A circuit arrangement including a cathoderay tube of the high Vacuum type having a cathode toemit electrons, an apertured cathode screen to form the emitted electrons into a beam, an apertured accelerating electrode comprising an apertured disc spaced from said cathode by a distance of between three and five times the diameter of theaperture in said apertured cathode screen, a decelerating electrode comprising an apertured diaphragm, a first anode, a second anode, and an electron receiving target, a source of electron beam modulating energy, electrical connections from said source to said cathode screen and to said cathode, potential means connected between said accelerating electrode and rection of said target, an apertured cathode. :screening disc in said cylindrical electrode on the target side of said cathode with its aperture axially aligned with said cylindrical electrode to focus the electrons emitted by said cathode into an electron beam, an apertured electron decelerating electrode between said cylindrical electrode and said target, an apertured electron accelerating electrode between and spaced by a distance of approximately one millimeter from said cylindrical electrode and said decelerating electrode,

means including an apertured anode to focus the electron beam on said target, the apertures of said screening disc, accelerating electrode, and anode being of substantially the same diameter and smaller than the diameter of said cathode, a;

direct electrical connection between said cathode and said decelerating electrode, potential means to maintain said accelerating electrode at a high positive potential with respect to said cathode, a source of signalling energy and electrical connections from said source to said cathode and to said screening I disc to modulate the electrons emitted by said cathode.

FREDERICK HERMES NICOLL.

BERNARD JOSEPH MAYO. 

